The alkali chlorine method, which is broadly carried out in the detoxification treatment of waste-water containing cyanides, has two steps for decomposing cyanides, i.e., an addition of chlorine under an alkaline condition; and, subsequently, a conversion of pH to neutral and a further addition of chlorine. A sodium-hypochlorite aqueous solution is usually used as the chlorine, and the dosage control by means of an ORP meter is carried out as follows.
First Step Reaction: pH 10˜12. The oxidation-reduction potential of 300˜350 mVNaCN+NaOCl→NaCNO+NaCN  (1)
Second Step Reaction: pH 7˜8. The oxidation-reduction potential of 600˜650 mV2NaCNO+3NaOCl+H2O→N2+3NaCl+2NaHCO3  (2)
According to “Techniques and Regulations of Pollution Prevention, Water Chapter, 5th Edition” supervised by the Environment and Location Bureau of the Ministry of Trade and Industry, pages 261˜262), the free cyano matter and cyano complexes of zinc and copper can be oxidized and decomposed by the alkaline chlorine method under dosage control using an ORP meter. Contrary to this, the cyano complexes of nickel and silver can be decomposed solely by prolonged reactions with excessive chlorine. Consequently, the chlorine must be dosed in a constant amount exceeding that required for decomposition. The dosing control by an ORP meter is, thus, impossible. Furthermore, since complexes of iron, cobalt and gold, which form stable complexes, are also stable under the presence of excessive chlorine, their decomposition is allegedly difficult by the alkaline chorine method.
The cyano matters are designated as harmful material in the Prevention Law of Water Pollution, and are defined as the total cyano matters including the cyano complex. Treatment methods of the cyano waste-liquor including these stable cyano complexes have been developed.
According to the method of Japanese Unexamined Patent Publication No. 49-1058, the cyano waste-liquor, which contains a cyano complex, is heated in a pressure vessel to 150° C. or higher so as to hydrolyze the cyano complex, and the dissociated free cyano matters are thermally decomposed into ammonia and formic acid. This method is referred to as the thermal hydrolyzing method. The formic acid, which is a decomposition product of this method, is a COD component of the waste liquor, and the ammonia is subjected to the nitrogen regulation. A secondary treatment is, therefore, necessary.
A method developed for obviating the need for the secondary treatment, is the wet oxidizing method (Japanese Unexamined Patent Publication No. 7-116672). In this method, an oxidizing process is additionally carried out during the reaction in the pressure vessel as described hereinabove. According to this method, the cyanides are oxidized and decomposed into carbon dioxide and nitrogen.
The two types of methods described hereinabove are based on physical chemical reactions and are therefore highly reliable in detoxifying cyanides. However, since each of these two types is a high-pressure treatment, the investment cost is enormous.
When the waste-water containing cyano complexes is treated by the thermal hydrolysis method or the wet oxidizing method, the metal, which is converted to a complex by the cyano matters, is precipitated and deposited in the form of hydroxide and oxide, resulting in the formation of sludge. In order to detoxify the waste-water, which is incorporated in the sludge, mechanical stirring becomes necessary. Installation of a stirring means in a high-pressure vessel incurs considerable increase of investment cost because the sealing characteristics of the vessel must be ensured. This is the reason that hinders the employment of those methods.
The present inventors carried out a document search in a broad range to find such treatment methods of waste-water containing cyano complexes that require minimum investment costs and are highly reliable. As a result, the present inventors focused on Japanese Examined Patent Publication No. 50-118962. This publication describes that waste-liquor containing an iron cyano-complex is heated and detoxified by means of dosing the oxidizing agent in a constant amount.
According to the method of Japanese Unexamined Patent Publication No. 50-118962, the pH value of waste-liquor containing an iron cyano-complex is adjusted to approximately 10˜11, an oxidizing agent (hypochlorite) is added, and the reaction is carried out while maintaining the liquor temperature at approximately 80˜95° C. Allegedly, the iron cyano complex is converted to iron oxide under the pH and temperature condition mentioned above and precipitates. Further, the cyano matters are decomposed into carbon dioxide and nitrogen.
However, notwithstanding a lapse of 25 years from the invention of Japanese Unexamined Patent Publication No. 50-118962, use of this method virtually has not spread. It can be said that persons skilled in this field do not recognize at all this method to be capable of treating cyano waste-liquor, which contains stable cyano complexes such as nickel, silver, iron, cobalt and gold complexes.
The present inventors made a tracing test and thus confirmed that the waste-liquor containing an iron cyano complex was detoxified as described in the specification.
The present inventors presume that the above method has not been put into practice for the following reasons.
{circle around (1)} Japanese Unexamined Patent Publication No. 50-118962 is the so-called chlorine treating method, in which the oxidizing decomposing always proceeds under an excessive amount of an oxidizing agent. The end point of the reaction is, therefore, not distinct. This method is, therefore, not reliable in the detoxifying the cyanides.
{circle around (2)} The hypochlorite present in excess self-decomposes at a temperature of 80° C. or more. The amount of the reagent used becomes therefore excessive.